Add traits for concat and connect methods
auto merge of #6908 : bjz/rust/concat-connect, r=Aatch

This adds the following traits to `prelude`:

In `std::str`, impled on `&[~[T]]` and `&[&[T]]`:
~~~rust
pub trait StrVector {
    pub fn concat(&self) -> ~str;
    pub fn connect(&self, sep: &str) -> ~str;
}
~~~

In `std::vec`, impled on `&[~str]` and `&[&str]`:
~~~rust
pub trait VectorVector<T> {
    pub fn concat(&self) -> ~[T];
    pub fn connect(&self, sep: &T) -> ~[T];
}
~~~